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arch, comprising the whole of the lower margin of the shell, with two 
remote projecting lateral teeth. Whether there exists in the other, as 
I believe, a ridge corresponding with the groove in this valve, or 
whether it was opposed by a corresponding groove, must remain to 
be ascertained by the examination of some more illustrative specimen. 
CXIY. Trigonellites. A slightly-rounded, trigonal, thick shell, 
gaping on each side. The anterior margin nearly on a straight line ; 
the posterior in a gently waving, and the upper side in nearly a 
circular direction. The outer surface of each valve thickly pierced 
by foramina, which, passing nearly through its substance, gives it 
the cancellous appearance of bone : the inner surface smooth, but 
marked with strife, concentric with the upper margin. The hinge 
completely linear, without teeth; there being only an appropriate 
surface, on the anterior margin of each valve, for the attachment of 
the cartilage externally. No appearance of muscular attachment. 
This shell, which I have presumed thus to name and to dispose of, is 
exceedingly anomalous in every respect : it is of an extraordinary degree 
of thickness, being, in some specimens, full three-eighths of an inch 
thick ; and, even in young specimens, full a quarter of an inch thick. 
A fossula, or at least a greater degree of concavity, is observable just 
beneath the beak, and which is the only part which bears the least ap- 
pearance of the animal having been attached to it. Possessing both valves, 
I have carefully sought to discover the state in which they would be, if 
united; but can only ascertain, that if brought into contact at the angular 
points, by approximating first the anterior sides of the valves, the shell 
gapes on each side, the rotundity of the shell occasioning a separation 
as the shell closes, of even the anterior margins, which are formed 
nearly on a straight line ; and which, when the valves are widely 
open, lay in a line with each other, as at Plate XIII. Fig. 10 and 11. 
If the superior margins, which alone can be thus brought into contact, 
are placed together, the shell is very widely separated at the beaks. 
This shell was first noticed by Scheuchzer as Concha fossilis 
